A representative optical directional coupler known conventionally is shown in FIG. 8. FIG. 8 is a schematic plan view of an optical directional coupler in the prior art. An optical directional coupler 50 shown in FIG. 8 has a substrate 51 disposed parallel to a plane where FIG. 8 is drawn, a first optical waveguide 52 and a second optical waveguide 54, these waveguides being composed on the substrate 51. The first and second waveguides 52, 54 have respective straight portions 56 disposed close to each other in a direction perpendicular to a light-transmitting direction 50a and defining an optical coupler portion. The first waveguide 52 has a first input port 58 and a second output port 60 while the second waveguide 54 has a third output port 62 and a forth input port 64. A coupling length L of the optical directional coupler 50 is considered hereinafter to be a length of the straight portion 56.
When light is transmitted from the first input port 58, locations where light strength is relatively strong exist alternately in the first and second waveguides 52, 54 along the transmitting direction 50a at a constant interference cycle or pitch. For example, in a condition in which a coupling length is determined so that light strength is relatively strong at the end of the second waveguide 54, most of the light is transmitted to the third output port 62 while a portion of the light is leaked to the second output port 60 as crosstalk. Thus, in an optical directional coupler, a splitting or branch ratio between lights output to the second and third output ports 60, 62 can be selected by adjusting or changing the coupling length. Light has polarized wave components TE, TM, the former being parallel to the substrate while the latter being perpendicular thereto.
FIG. 9 is a graph showing amounts of crosstalk of the respective polarized wave components. In FIG. 9, a horizontal axis indicates half of the coupling length L while a vertical axis indicates an amount of crosstalk or excess loss. FIG. 9 is a graph showing a state where most of the light transmitted from the first input port 58 is output to the third output port 62. In this state, since light output to the second output port 60 is crosstalk, excess loss at the second output port 60 is preferably large, i.e., an absolute value in a negative decibel value of excess loss is preferably large, while excess loss at the third output port 62 is preferably small, i.e., a decibel value thereof is preferably close to zero. As can be seen from FIG. 9, values of excess loss of the polarized wave components TE, TM at the third output port 62 do not substantially change when the coupling length L is changed. On the contrary, values of excess loss (crosstalk) of the polarized wave components TE, TM at the second output port 60 are affected by a difference between phases of the polarized wave components because a very small amount of light is transmitted to the second output port 60. For example, when a coupling length is selected to minimize a value of excess loss of the polarized wave component TE, a value of excess loss of the polarized wave component TM cannot be the smallest. Thus, it is impossible for values of excess loss of the both of the polarized wave components TE, TM at the second output port 60 to be smaller than −25 dB.
Patent Publication 1 indicated below discloses an optical directional coupler having straight waveguides close to each other, in which widths of the straight waveguides are different from each other in order to reduce fluctuation of excess loss due to a difference in the types of polarized wave. Further, on the opposite sides of the narrower straight waveguide, tapered optical waveguide portions for conforming widths of the tapered waveguide to those of the input and output ports are provided coaxial to the narrower straight waveguide.
Patent Publication 1: Japanese Patent Laid-open Publication No. 6-308338 (Please refer to FIG. 1) An amount of the above-stated crosstalk is desirably as small as possible. In order to reduce the amount of crosstalk, phases of the polarized wave components TE, TM should be brought close to each other.
Further, in the optical directional coupler disclosed in Patent Publication 1, since widths of the straight waveguides thereof close to each other are different from each other and thus the optical directional coupler is asymmetric, it is difficult to make an optical circuit with a splitting or branch ratio of 0:100% in which all light input from the first input port 58 is transmitted to the third output port 62. Namely, when directional couplers having a splitting or branch ratio of a few tens % in a broad frequency range are required to be made with a great yield ratio, such a directional coupler, in which widths of the straight waveguides are different from each other as disclosed in Patent Publication 1, is selected. Although such a directional coupler has a property depending on the type of polarized wave, an influence thereof is very small because an amplitude of a strength-transition curved line (sinusoidal curved line) of transmitting-light is made small and upper and lower peaks of the amplitude are utilized. Namely, a structure in which widths of the straight waveguides are different from each other is only a little affected by the type of polarized wave and interference lengths or pitches themselves which differ depending on the type of polarized components, and thus these lengths or pitches are not adjustable.
It is therefore an object of the present invention to provide an optical directional coupler which enables a phase property of polarized wave components of transmitting-light to be changed.